3. Some uses of A-B translocations. The B chromosome provides a centromere to which specific segments of A chromatin may be translocated. The exceptional behavior of the resultant chromosome in the second microspore division provides a mechanism for the accumulation of this chromosome for various cytogenetic problems in which applications are useful. One application of this, now in progress, is a study of the effect of accumulation on the phenotype of recessive and intermediate alleles, using T2-B for a comparison of B, Bw, and b in various doses.
The fact that A-B translocations produce functional gametes deficient for as much as a whole arm of an A chromosome provides a tool for the location of recessive genes in the physical chromosome in a single generation. One would simply cross known A-B translocations on the recessive in question. If the locus of this gene is in the translocation chromosome with the B centromere, the recessive phenotype will appear in the F1. For example, if the recessive is located in the distal four-fifths of the short arm of chromosome 4, it will appear in the F1 of a cross by T4-B. The results summarized in the table place Su in this region. Likewise Gl and Ij are in the distal two-thirds of the long arm of chromosome 7, whereas O2 is not in this segment. An extensive planting for new A-B translocations involving different segments of the A chromosomes is planned for this summer.